Adjustable sliding brush transformer



Sept. 9, 1969, R. A. KRoL ADJUSTABLE SLIDING BRUSH TRANSFORMER OriginalFiled Feb. 6, 1967 fig.;

INVENTORI United States Patent O 3,466,583 ADJUSTABLE SLIDING BRUSHTRANSFORMER Ronald A. Krol, Fort Wayne, Ind., assignor to GeneralElectric Company, a corporation of New York Continuation of applicationSer. No. 614,218, Feb. 6, 1967. This application Sept. 18, 1968, Ser.No. 768,594 Int. Cl. H011. 27/08 U.S. Cl. 336-60 2 Claims ABSTRACT OFTHE DISCLOSURE Improved adjustable 4sliding brush transformer having acoolant passageway between the brush track and brush supportingstructure to reduce the transformer hot spot temperature. The brushsupporting structure has a bifurcated portion. The bifurcated portiondefines an outlet for the discharge of convective air currents in anaxial direction away from the *brush to reduce the hot spot temperatureof the transformer. The brush supporting structure includes a -guidewayto maintain alignment of the brush relative to the brush track and toprevent binding of the brush in the brush supporting structure duringoperation.

. This is a continuation of application Ser. No. 614,218 filed Feb. 6,1967, now abandoned.

This invention relates to adjustable sliding brush transformers, andmore particularly to improvements therein whereby the hot spottemperature of the transformer is substantially reduced.

Variable transformers of the sliding brush type typically have anautotransformer coil wound on a core with a surface portion of the coilforming a brush track. A brush consisting of one or more brush segmentscarried by a brush holder slides across and makes electrical contactwith the brush track. The output current of the transformer is suppliedfrom the winding through the brush to any desired external load. Theoutput voltage of the transformer is varied by the selective positioningof the brush along the brush track.

:In practice it often happens that the amount of current that can besupplied to a load and therefore the current rating of the transformerislimited by the maximum allowable temperature at the interface of thebrush and brush track. The hot spot temperature, which is the highesttemperature occurring at a point along the interface, is used inpractice vto determine the current rating of a transformer design. y

When the hot spot temperature exceeds certain predetermined limits, thebrush to brush track resistance increases heat generated at theinterface. With continued operation under this condition the transformermay prematurely fail.

Another factor influencing hot spot temperature has been related to thealignment of the brush relative to the brush track. If the brush becomesmisaligned, the interface area between the 'brush and brush track willbe reduced and during operation the current density flowing through thecontact face of the brush will correspondingly increase. With increasedcurrent density the hot spot temperature will also increase.

In view of the hot spot temperature problem, manufacturers of variabletransformers have found it necessary in some applications to reduce therated output current for a given unit. It will thus be appreciated thatthe maintenance of proper alignment and contact between the brush andtrack in a variable transformer and of the lowest possible hot spottemperature is extremely desirable.

Patented Sept. 9, 1969 Accordingly, it is a general object of thepresent invention to provide an improved adjustable sliding brushtransformer characterized by relatively lower hot spot temperatures.

Another object of the present invention is to provide an improvedsliding brush transformer wherein means are provided to promote andmaintain proper alignment between a brush and brush track in thetransformer.

Yet another object of the invention is to provide an improved slidingbrush transformer wherein convection of air axially away from the the'brushes is achieved to reduce the hot spot temperature.

A still further object of the invention is to provide an improvedadjustable brush transformer in which the above objects are obtained andwhich can be manufactured relatively easily and inexpensively.

In accordance with one form of the invention, I have provided animproved adjustable sliding brush transformer in which a brushsupporting structure holds a brush against a brush track on thetransformer winding. In the preferred embodiment of the invention, thebrush supporting structure includes a brush arm having a bifurcatedportion. The brush arm forms a cooling channel with a portion of thebrush track and the bifurcations of the brush arm define an outlet forconvective air currents communicating with the cooling channel. Theoutlet overlies portions of the brush and air can flow through theoutlet axially away from the brush to reduce the hot spot temperature ofthe transformer. The brush arm in the preferred embodiment of theinvention also includes means that guide the brush and maintain adesired orientation of the brush relative to the brush track.

The subject matter which I regard as my invention is set forth in theappended claims. The invention itself, however, together with furtherobjects and advantages thereof may be best understood by referring tothe following description taken in `connection with the accompanyingdrawing in which:

FIGURE 1 is a perspective view of an adjustable sliding brushtransformer embodying one form of the invention;

FIGURE 2 is an exploded view in perspective of a brush arm assembly usedwith the adjustable sliding brush transformer shown in FIGURE 1;

FIGURE 3 is a plan view of a portion of the transformer shown in FIGURE1 and illustrates the relation- 'ship of the brush and outlet formed bya bifurcated brush arm; and

FIGURE 4 is a sectional view of a portion of the transformer of FIGUREl.

Referring now to the drawing, my improved adjustable transformer,generally identified by numeral 18, comprises a core 19 of magneticmaterial substantially encased by an insulating material 23. Woundaround the core of the transformer is a winding 20 of insulated wire. Aportion of a surface of the winding has the insulation removed therefromto form a brush track 21. The core and coil assembly of the transformer18 may be bonded within the wall 27 on the base 21 with a resinuousmaterial 25 in the manner taught in United States patent applicationSer. No. 582,024, filed Sept. 26, 1966 by Winston R. Iudd and assignedto the assignee of the present application.

In the preferred embodiment, the brush holder assembly 28 is attached tothe radial member and the brush holder 36 resiliently urges a pluralityof brush segments 56 into electrical contact with the brush track 21.The radial member 24 has ventilation openings 26 therein as described inthe above mentioned Judd application, and a control knob 30 is providedfor manually setting the location of the brush on brush track 21.

When the transformer is used to supply current to a load, the outputcurrent flows from the winding through the brush segments 56, the brushsupporting plate 62, the brush holder 36, and a conducting ring 29 asdisclosed and claimed in U.S. Patent No. 3,160,841 owned by the assigneeof the present application. A brush mounted in housing 16 bears againstthe conducting ring 29 and is connected to the output terminal 13 on theterminal plate 10. It will be appreciated that the provision ofterminals l11, 12, 14 and 15 on the terminal plate 10` in the mannershown provides iiexibility in selecting the desired range of voltage forapplication to the load.

The brush carrying structure includes the radial mem- 'ber 24, brushholder 36, and brush assembly 54. As shown in FIGURE 2, each brushsegment 56 is biased away from the backing plate 62 by a spring 58. Thebrush conductors 60, having one end embedded in a brush segment 56 andanother end thereof secured to the backing plate 62 by suitable meanssuch as a spot weld, hold the springs 58 in a slightly compressedcondition. Holes 63 in the backing plate 62 are aligned with the holes33 in the brush arm 36 and screws hold the plate and arm in assembledrelation. After assembly, the brush segments 56 are slidably disposedwithin the guide way 39 of the brush holder 36.

The bifurcated brush holder 36 includes two bifurcations 40, 42 and twoleg portions 34 which form opposite sides of a brush receiving guide way39. Preferably sufficient clearance between the brush and brush guideway 39 is provided so that the brush may be continually biased againstthe brush track 21 during operation. However, the clearance should bekept as small as possible so that the brush does not vibrate duringoperation and cause arcing between the brush and brush track. In orderto maintain proper alignment of the brush segments 56 relative to thebrush track 21 and prevent binding of the brush in the guide way 39 dueto thermal expansion and thermally induced warping of the leg portions34, the bridging section 38 is provided between leg portions 34 of thebrush arm 36. It will thus be appreciated that the bridging section 38provides dimensional stability for the guide way 39 during operation.

Now having reference to FIGURES 3 and 4, it will be seen that the brushholder 36 is held in spaced relation to the brush track 21 duringoperation to form a cooling channel between the brush holder 36 andbrush track `21. As can be seen in FIGURE 3, portions of the brushsegments 56y are positioned in the cooling channel and lie directlybelow a coolant outlet defined by bifurcations 40, 42, the surface 37 ofbrush holder 36 and the conducting ring 29. As shown in FIGURE 4, acooling medium such as forced or convective air currents will flow inthe direction of the arrow axially along and axially away from the brushsegments 56 through the outlet. It will also be seen from FIGURE 4 thatcoolant can escape radially outwardly from the transformer acrosssurface 37 of the brush holder 36.

In the actual reduction to practice of the invention the brush holder36- was die cast zinc. The bifurcated portions 40, 42 of the brushholder 36 had recessed surfaces 46, 48 so that the heads of mountingscrews used to attach the brush holder to the radial member 24 did notproject beyond the brush arm 36. The tolerance between the guide way 39and the brush segments 56 was less, from about .0005 inch to about .003inch, which was sufficient to permit the springs 58 to bias the brushsegments 56 into good electrical contact with the brush track 21 duringoperation but was also sufficiently small to prevent cocking of thebrush segments 56 within the guide way 39 which could cause misalignmentof the brush with respect to the brush track.

Tests were run on a variable transformer with and without an improvedconvective air flow outlet brush holder arrangement to obtaincomparative data relating to hot spot temperatures. In order to reducethe variables in the test, the same core and coil unit as well as thesame load was used in each test. To further reduce variables in thetests the same brush holder and brush, similar to the exemplified brushholder and brush, were used in each test. However, during the first testthe improved convective air flow outlet brush holder was temporarilymodified by closing the outlet with putty in order to approximatelysimulate a transformer arrangement without an air flow outlet.

At the beginning of the second test run the putty was removed, with carebeing taken not to disturb the brush setting, so that the second set oftest data would indicate the range of hot spot temperatures present in atransformer having the improved convective air flow outlet brush holderarrangement.

During both tests, temperature indicating paint, stated to have atemperature indicating accuracy of one per cent, and manufactured by theTempil Manufacturing Company of New York was used to obtain anindication of hot spot temperatures. As is understood by persons skilledin the art, different formulations of the Timpil paint is manufacturedand individual formulations exhibit pigmentation changes when heatedabove the predetermined temperature for each particular formulation.

In both test, 236 volts were applied across terminals 11 and 14 and 30amperes of current at 120 volts were supplied to the load from terminals11 and 13. The output voltage of l2() volts corresponded to anapproximately centered position of the brush on the brush track. Afterthe transformer had been operated continuously for over twenty hours ineach test, successive formulations of the Tempil paint were applied tothe brush track adjacent the brush and observed for pigmentationchanges. The temperature sensitivity of each successive Tempil paintformulation differed from the preceding formulation by three to fourdegrees centigrade, and it was therefore possible to bracket within afew degrees a temperature which was indicative of the hot spottemperature during each test.

The results of the first test showed that in the transformer without animproved convective air flow outlet brush holder the hot spottemperature was between 173 and 177 degrees centigrade. However, thesecond test showed that in the exemplified transformer with an improvedconvective air flow outlet brush holder the hot spot temperature wasbetween 142 and 146 degrees ccntigrade. It will thus be appreciated thatnearly an 18 percent reduction in hot spot temperature was observed inthe transformer embodying the convective air ow outlet aspect of thepresent invention.

It will also be appreciated that by reducing the hot spot temperature invariable transformers, the output current rating for a given frame sizeand core and coil size can be increased.

From the foregoing description it will now be appreciated that I haveprovided an improved variable transformer of the adjustable brush typewhich is particularly characterized by reduced hot spot temperatures inthe transformer. It will also be appreciated that I have provided asolution for the problem of brushes binding in brush holders as well asan improved arrangement wherein the alignment between a brush and brushtrack can be maintained in practice.

While I have illustrated and described a particular exemplification ofmy invention, it will be apparent that many modifications may be madeyas will be apparent to those skilled in the art. It is to be understoodtherefore that this invention is not intended to be limited to theparticular embodiment shown and that it is intended by the appendedclaims to cover all modifications that come within the spirit and scopeof this invention.

I claim:

1. In an adjustable sliding brush transformer cornprising an annularcore and coil assembly having a substantially vertically extendingcentral axis and a brush track therearound, a brush resiliently engagingthe brush track and making sliding electrical contact therewith, theimprovement in said transformer comprising: brush carrying structurehaving means for guiding the brush to maintain a predeterminedorientation of said brush relative to said brush track, and supportmeans supporting said brush carrying structure in spaced relation to thebrush track to form a cooling channel between said brush carryingstructure and brush track, said brush carrying structure comprising abifurcated portion for attachment to said support means, and saidbifurcated portion having a pair of bifurcations extending substantiallyradially inwardly toward said central axis and said pair of bifurcationsdehning an outlet communicating with said channel and overlying portionsof the brush engaging said brush track whereby convective air currentsmove past the brush track and brush and discharge in an axial directionaway from said brush through said outlet to thereby reduce the hot spottemperature of the brush.

2. A sliding brush transformer comprising an annular core and coilassembly having a substantially vertically extending central axis and abrush track surface, a brush for making uniform sliding electricalcontact with said brush track surface, brush supporting structure forcarrying said brush and maintaining a predetermined orientation of saidbrush relative to said brush track, means for mounting said brushsupporting structure in spaced relation to the brush track to form acooling passageway therebetween, said means for mounting said brushsupporting structure including a bifurcated portion, said bifurcatedportion having a pair of bifurcations extending substantially radiallyinwardly toward said central axis and said pair of bifurcations definingtherebetween an outlet for convective air currents communicating withsaid cooling. passageway and overlying portions of said brush wherebyconvective air currents flowing axially along the brush dischargethrough the outlet in an axial direction away from the brush thereby toreduce the hot spot temperature of the transformer, means to preventbinding ofthe brush in the brush supporting structure during operation,and means cooperating with said brush supporting structure and saidbrush for resiliently urging said brush into electrical contact with thetrack.

References Cited UNITED STATES PATENTS 2,089,434 8/ 1937 Schermerhorn336-149 XR 2,265,666 12/1941 Mekelburg et al. 336-149 XR 2,388,99611/1945 Preston 336-149 LEWIS H. MYERS, Primary Examiner T. J. KOZMA,Assistant Examiner U.S. C1. X.R.

